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Learning

Channels

by James Ritson
Learn what Channels are in image editing and what you can do with them.

Channels make up the luminance and chrominance information in an image: think of them as building blocks for light and colour. Every image contains channel information, and the number and layout of these channels depends on the image’s colour format.

RGB contains three channels - Red, Green and Blue

CMYK contains four channels - Cyan, Magenta, Yellow and Key (Black)

LAB contains three channels - Luminance, A Opponent and B Opponent

Greyscale contains one channel - Intensity

The three colour channels (represented in greyscale) that make up the image’s colours.

These colour formats are all ways of representing colour and light to create the final image.

RGB is the most commonly used format for computer imagery. It’s an additive format, which means that the red, green and blue channels are added together to produce the final image.

CMYK is used for printing. It’s a subtractive format—you start with the absolute white of the paper and subtract away from that, mixing cyan, magenta, yellow and black.

LAB is designed to closely mimic human visual perception and is predominantly for professional image work. Its colour space exceeds that of RGB and CMYK, which is beneficial if work is destined for both screen and print—sRGB may contain values that cannot be reproduced in CMYK and vice versa, but working in LAB ensures you edit in a gamut that covers both, meaning conversions can be more accurate.

Greyscale contains no colour information—instead, its one channel is used to dictate the intensity of the pixels from black to white.

Working with channels

Manipulating channels separately allows for a number of advanced editing techniques, such as permitting more controlled tonal adjustments, creating accurate selections based on colour, and masking adjustments and filters to specific colour channels.

Such techniques include:

  • Painting onto individual channel layers using raster tools for isolated tonal adjustments and modifications such as cloning, healing, dodging and burning.
  • Creating masks from channels, allowing adjustments and filters to be applied to specific channel information.
  • Copying greyscale channel information to a new raster layer, producing a greyscale image that can be used creatively for blending or composite work.
  • Saving selections as channels, enabling them to be re-used at a later date.
Colour alteration

In this image, a selection is made based on the red channel: a Hue/Saturation/Lightness adjustment is then added and masked to this selection. The red channel’s saturation is then increased, allowing the tone of the leaves to be intensified without over-saturating the rest of the image.

Selection refinement

Channels are incredibly useful for advanced selection techniques, as individual channels can provide better tonal separation.

The image’s blue channel contains the most contrast, which makes it ideal for making a clean selection of the subject.

The Blue channel of the image, which contains the most contrast between foreground and background.

The blue channel is extracted to a greyscale layer, and selection refinement is used to achieve an accurate, matted selection.

The Blue channel as a greyscale layer being used for selection refinement.

A more accurate selection results in a cleaner mask, making it easier to cut the subject out for compositing.

Storing selections
Storing a selection as an additional channel on the Channels Panel.

Selections can be saved as spare channels, allowing that selection information to be loaded again in the future as either a pixel selection or a mask layer. This can help speed up workflows as you can store multiple selections—perhaps variations—removing the need to start repeat selections from scratch.

Creative channel mixing

Channel information can be mixed, deleted and swapped, allowing for creative techniques such as emulating two or three strip film processes like Technicolor.

Three-strip Technicolor emulation using channel techniques.
Channel swapping

In addition to mixing channel information, certain techniques like infrared processing benefit from swapping entire channels—many infrared workflows involve swapping red and blue (known as a two-channel swap) or red and blue plus blue and green (a three-channel swap, where the green channel is removed entirely and replaced by the blue channel information).

An infrared image after its red and blue channels have been swapped.

Affinity and channels

Affinity Photo has full support for channels in all supported colour modes (RGB, CMYK, LAB, Greyscale) and allows individual channel previewing, editing, copying and masking.

We have a wide range of Affinity Photo tutorials pertaining to channels: